Four methods of family planning are currently available in the U.S., sterilization, abstinence, abortion and contraception. Of these four birth control methods, contraception is the most widely utilized. Despite the substantial U.S. and global demand for contraception, the presently available methodologies fall short of market needs. Oral contraceptives and barrier methods dominate today's contraceptive market but have significant shortcomings. Oral contraceptives, though efficacious, are documented to be associated with significant side effects including increased risks of cardiovascular disease and breast cancer and are not recommended for women over the age of 35. Barrier methods, while safe, have failure rates approaching 20%. There is a clear need for increased availability of and improvements in contraceptives that offer superior safety, efficacy, convenience, acceptability and are affordable to women and men worldwide. Identification of novel approaches for controlling fertility is therefore necessary.
Immunization of male and female animals with extracts of whole sperm is known to cause infertility. Tung, K., et al., J. Reproductive Immunol., 1; 145-158 (1979); Menge, A., et al., Biol. of Reproduction, 20, 931-937 (1979)!. Moreover, men and women who spontaneously produce antisperm antibodies are infertile, but otherwise healthy. Bronson, R., et al., Fert. and Sterile, 42, 171-183 (1984)!. Although the critical sperm antigens are unknown, these observations have led to the proposal that sperm proteins might be useful in the development of a contraceptives vaccine.
In mammalian species, sperm proteins are believed to have a role in sperm adhesion to the zona pellucida of the egg. The PH30 protein is known to be involved in sperm egg binding and antibodies that bind to PH30 inhibit this interaction. PH30 is an integral membrane protein present on posterior head of sperm which mediates sperm-oocyte fusion. The PH30 protein consists of two immunologically distinct alpha and beta subunits. Both subunits are made as larger precursors and then finally processed in epididymis where sperm become fertilization competent. Primakoff, P., et al., J. Cell Biology, 104, 141-149 (1987); Blobel, C. P., et al., J. Cell Biology, 111, 69-78 (1990)!. Monoclonal antibodies that recognize PH30 inhibit sperm-oocyte fusion in vitro, indicating its importance in fertilization Primakoff, P., et al., J. Cell Biology, 104, 141-149 (1987)!.
Guinea pig PH30 alpha and beta chains have been cloned by Blobel et al. Mature PH30 alpha chain consists of 289 amino acids and encodes a transmembrane domain as well as an integral fusion peptide (82-102) that is similar to a potential fusion peptide of E2 glycoprotein of rubella virus. Guinea Pig PH30 beta chain has an open reading frame of 353 amino acids and also encodes a transmembrane domain. Blobel C. P., et al., Nature, 356, 248-251 (1992)!. The predicted amino acid sequence of the PH30 beta chain protein contains significant homology to a class of proteins called disintigrins found in snake venom. These proteins are known to bind to a family of proteins called integrins and prevent their normal functioning in cell adhesion (a well studied example is platelet aggregation). The N-terminal ninety amino acids integrin binding disintigrin domain of PH30 beta has been postulated to mediate the binding of PH30 to its putative integrin receptor on oocytes. The cloning and sequence determination of the mouse and human PH30 beta chain genes would permit novel approaches to the control of sperm egg binding and fusions. These approaches include, but are not limited to, eliciting an immune response directed at all or part of the PH30 beta chain protein and using the PH30 beta chain protein as part of a screen to identify small molecules that alter sperm egg interactions.
Mammalian fertilization is, in most cases, species specific. Thus, the identification and isolation of sperm surface proteins essential for fertilization in species other than guinea pig would be useful for providing effective long lasting contraception in those species. Thus far, the lack of biochemical identification, isolation and cloning of candidate adhesion proteins of sperm has hindered scientists in developing effective contraceptives for humans as well as other mammalian species.